Method in soda recovery boiler plants for re-feeding dust discharged from an electrostatic precipitator



1964 H. ANDERSSON ETAL 3,161,464

METHOD IN SODA RECOVERY BOILER PLANTS FOR RE-FEEDING DUST DISCHARGED FROM AN ELECTROSTATIC PRECIPITATOR Filed April 18, 1961 INVENTORS HARRY ANDERSSON COSTA Ml LDH ATTYS.

United States Patent Ofiice 3,161,464 Patented Dec. 15, 1964 3,161,464 METHOD IN SODA RECOVERY BOILER PLANTS FOR RE-FEEDING DUST DISCHARGED FRDM AN ELECTROSTATIC PRECIPITATGR Harry Andersson, Stockholm, and Giista Mildli, Vattersnas, Sweden, assignors to Aktiebelaget Svenska Fiaktfabriken, Stockholm, Sweden Filed Apr. 18, 1961, Ser. No. 103,754 1 Claim. (Cl. 213-48) The present invention relates to a method in soda recovery boiler plants for re-feeding dust discharged from an electrostatic precipitator, such as a hot-precipitator. In soda recovery boilers of the injection type the natrium sulphate necessary for compensating the loss of alkaline in the process is mainly added by being mixed into the lye concentrated by evaporation before the injection of same. This addition of natrium sulphate causes a heavy wear of the injection pumps and injection nozzles which necessitates a repeated exchange of said means with short intervals, resulting in an increase of the running costs per year of a size of about 15,000-25,000 crowns for a middle sized plant. Another drawback is that-fr making the intermixing possible-it has proved necessary to choose a dryness of the lye which from the point of heat economy is unsuitably low. During the intermixing procedure furthermore a decrease of the temperature of the lye takes place, which besides a reduction of the heat content of the lye also causes an increase of the viscosity and thus renders the atomizing of the lye in the combustion chamber more diflicult.

The invention, the object of which is to eliminate the above mentioned drawbacks, is characterized in that the dust is granulated by being compressed or subjected to another mechanical treatment and in this condition is fed directly into the combustion chamber of the boiler. The granulation of the dust is suitably carried out in direct connection with the discharge of the dust from the precipitator by means of a scraper conveyor. The granulation of the dust is moistened and thereafter in a manner known per se is mechanically worked for instance between knurled rolls or so called double paddle conveyors.

The invention involves besides the elimination of above mentioned drawbacks also the feature that the transport of the discharged dust can be carried out by the aid of simple means which need less space and without any risk of interruptions due to the clogging of the transport ways. i

The invention will now be more specifically described with reference to the accompanying drawing illustrating a suitable arrangement for the performance of the invented method.

In the drawing 1 designates a soda recovery boiler at the bottom of which the practically dry lye forms a layer 1a at the surface of which the primary combustion and the deoxidizing of natrium sulphate to mainly natrium sulphide takes place. Designation number 2 shows an electrostatic precipitator for separation of the main part of the dust suspended in the flue gases from the boiler. In the case shown the title gases are conducted from the precipitator to a heat recovery unit 3 (economizer) of known design. These are, however, no hindrance for disposing said precipitator behind the economizer. The dust falling down during the rapping of the collecting electrodes of the precipitator is transported by means of a scraper conveyor 4 located in the lower part of the precipitator chamber to a discharge trough 5 provided with a screw conveyor or another transporting means. Said means feeds the dust via a sluicing device 6 and a chute '7 down to an apparatus 8 for granulating the dust. In the case shown said apparatus consists of two rolls shaped in a suitable manner by means of which the dust is compressed and formed into a size and shape suited for the injection into the boiler. The dust being formed in this way is fed by means of an injector 9 into a supply duct 10 provided with one or more nozzles and connected to a transporting fan 11. The fan 11 can work with room air, part of the combustion air or flue gases. By the invention there is gained the advantage that the dust can be recirculated into the process directly, i.e., Without first intermixing the dust into the lye. Due to the granulation, said recirculation can take place Without any risk that considerable part of the dust may be caught by the gases from the layer and circulated in the system, resulting in a continuous increase of the dust concentration to amounts jeopardizing the performance of the process. Furthermore, the granulation prevents the particles from reaching such a size that there will be difiiculties for the chemical deoxidizing on the surface of the layer. It is essential for obtaining a proper reduction that the particles be of such a size that they can be spread over a great part of the area of the layer by a suitable adjustment of the injection velocity and the design of the supply duct.

We claim:

The method of recovering combustion gas dust from a soda recovery boiler plant which comprises, passing the combustion gases and dust from the fuel bed of the boiler past heat exchange elements and thence through a dust precipitator of the type which collects very fine particles in the dry state, collecting the dust from the precipitator, moistening and compacting the collected dust, granulating the compacted wet dust material, and feeding the granulated dust particles directly while moist and granular back upon the fuel bed in the boiler.

References Cited in the file of this patent UNITED STATES PATENTS 

